THE MEASUREMENT OF RESISTANCE 



163 



his invention " A Differential Arrangement." The variable ratio 

 arms were added by Dr. Werner Siemens. In 1843 Sir Charles 

 Wheatstone recalled attention to Christie's device, giving him full 

 credit. At that time Wheatstone was one of the leading scientists 

 of Great Britain, and his name became associated with the instru- 

 ment and has so remained. 



Having the conductors arranged as in the diagram, Fig. 89, 

 the current through the galvanometer will be zero only when 



ir = p , for to have zero current in the galvanometer, the poten- 

 tial difference between the galvanometer terminals must be zero, 

 or, in other words, the fall of potential along M must be equal to 

 that along X, and the fall along N equal to that along P. 



M 



N 



FIG. 89. Diagram for Wheatstone bridge. 



Let I M , IN, Ix, IP be the unvarying currents in the respective 

 branches; then when I G = 0, MI M = XI X , also NI N = PI P , 



MI M _ XIx 

 NI N ~ PI P 



but if no galvanometer current flows, 



I M = I N and I x = IP 



so 



_ 



N - p or X - 



Consequently, if three of the resistances are known, the fourth 



